Passenger aircraft could be fitted with an extra level of cockpit security if laws proposed in the United States are passed.

US lawmakers are pushing for stronger aviation security, which would include forcing airlines to install secondary security doors between the cabins and cockpits on all aircraft to prevent 9/11-style hijacking attacks, Reuters reports.

Despite the many developments in aircraft security since the terror attacks of September 11, 2001, when hijacked planes flew into New York's World Trade Centre and the Pentagon, four US representatives - Democrats Andre Carson and Josh Gottheimer and Republicans Brian Fitzpatrick and Peter King – said in a news release that hijackings were still a threat.

Last year, the US Congress imposed a requirement for security barriers in newly manufactured aircraft, which would keep the flight deck secure cockpits if pilots had to leave for a bathroom or meal break.

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The new bill, introduced last week, would extend these requirements to existing passenger jets.

The proposed secondary barriers would put a second door between the cockpit door and the rest of the plane.

The bill is called the Saracini Enhanced Aviation Safety Act – named after pilot Victor Sacarini, who was killed when his plane was hijacked during the 9/11 attacks.

His widow, Ellen Saracini, has since become an advocate for aviation safety.

"It is unacceptable that, more than 17 years after terrorists breached the cockpit of my husband's airplane on September 11, 2001, our skies are still susceptible to repeat this act of terrorism," she said in a statement.

"It is my mission to ensure we are doing everything we can to protect the flight deck aboard our nation's airliners because, without secondary barriers, we are just as vulnerable today as we were on that fateful day."

According to Reuters, current measures to prevent hijackers rushing the cockpit included stationing a flight attendant or food cart in front of the door.

A study by the Federal Aviation Administration (FAA) – which oversees aviation security in the US – found cockpits to be vulnerable in the moments when pilots needed to step outside, according to the representatives' news release.

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It said secondary doors were the most efficient and cost-effective solution to the problem, with lawmakers saying it would cost around US$5000 ($7400) to US$12,000 ($17,800) to install lightweight, mesh barriers in a single aircraft.

However, industry trade group Airlines for America – which represents major carriers like American Airlines, Southwest and United – said it should be up to individual carriers to decide whether to install such systems.

Association spokesman Vaughn Jennings told Reuters that the airline industry had already worked closely with the US Transportation Security Administration (TSA) to implement increased security systems following 9/11 and said airlines had found secondary cockpit barriers to be appropriate on some aircraft.

The US pilot union, The Air Line Pilots Association, said it supported the legislation.

Aircraft doors were reinforced and airport security screening was ramped up by the TSA after the 9/11 attacks.

The Federal Air Marshal Service also places armed US air marshals on flights around the world and is supervised by the TSA.

However, according to Reuters, some critics doubt the effectiveness of passenger screening, as well as the air marshal program.